However, Volvo isn't using the tech just to improve performance; it is using the tech to improve fuel efficiency. According to the automaker, KERS can reduce fuel consumption as much as 25 percent. In addition to improving fuel economy, Volvo says that the KERS tech can also reduce production costs compared to traditional hybrid systems.

Prototype Volvo S60 with Flybrid KERS system

Volvo has test fitted what it calls a “Flybrid” KERS to the rear axle of a S60 to assist the gas engine that drives the front wheels of the car. It captures kinetic energy typically lost from braking and sends it to the flyweel. 150 watt hours of energy can be captured in only 8 seconds, and the energy can be stored for up to 30 minutes or used immediately.

The KERS system – which spins its flywheel at a maximum of 60,000 rpm in a true vacuum and can deliver 80hp -- can be used to knock 1.5 seconds off the car's 0-60 time or in an economy mode to reduce pollution.

The entire system only weighs about 130 pounds, which makes it much lighter than a traditional electric motor and NiMH/Li-ion battery packs used in hybrid vehicles. For example, the batteries alone used in Volvo's current hybrids weigh 660 pounds.

Flybrid KERS (Kinetic Recovery System)

The prototype KERS systems won’t reach production in its current form. A production version of the system will see a similar flywheel and transmission attached to a front wheel drive-based transmission.

I am not sure of the process you used to come up with the 20kg battery pack, but I feel like you may be over-simplifying a bit here.

Apart from the weight of the battery and e-motor, the battery support (heating,cooling, charge management) and the high voltage wiring need to be taken into account. This is a lot of complexity that doesn't really exist in the flywheel concept and complexity typically adds cost.

Then you have to consider the longevity of the battery, it will be difficult to find a battery that can be completely charged/discharged in 8 seconds and still have an acceptable usable life.

Comparing a flywheel system like this to a plug-in hybrid is missing the point in my opinion, and is really an apples to oranges comparison. PHEV are meant to power the vehicle solely by electric power for miles at a time, the system here is meant to capture the energy wasted during braking and use it to augment the acceleration.

Being able to significantly boost efficiency, over a non-hybrid, with a pure mechanical system that weighs less than a 150 lbs is pretty cool.

KERs is far more complex than batteries and wires. There is no way to maintain 60k rpm in a consumer product subject to wildly varying temperatures and shock over time. This is a waste of resources, and will never be viable. It will only be bought as toys for those who got lucky standing underneath a bernanke helicopter drop. No one who actually works for a living is ever going to find this design economical.

KERS, as it is used in the article, is kinetic energy recovery system, it captures the energy of the moving vehicle. KERS is used in all hybrid or pure EV vehicles, not just flywheels.

"Batteries and wires" is a gross over-simplification. This isn't lighting a Christmas light with a 9V battery. All modern batteries need a support system to allow them to survive "wildly varying temperatures and shocks overtime".

Any turbo engine on the market today has a turbine that spins at 10's of thousands of RPM so RPM itself is not an issue. The 60K RPM was peak for the system and is only reached when the vehicle is slowing or accelerating.

What makes you think this is more expensive than a battery? Look at the price of used HEV's, the car is worth less than the cost of a battery replacement. How is that economical?

I didn't say it will entirely charge in 8s. I said it will charge 150 Wh. If you put a large enough battery in there then it can do that.

Take a look at A123's 32113 cell, which has been around for a few years now (I'm sure better batteries exist). It weights 205g, and will do 500W charge/discharge for 10s. It's a 15Wh cell that will do over 250kWh of energy throughput before degrading to 80%. Take 100 of these cells, and the pack will be able to take hundreds of thousands of 150Wh microcharges before degrading.

The flywheel isn't cheap and simple either, as you need a CVT and sophisticated control to transfer the energy seamlessly to and from the 0-60k RPM flywheel. Its reliability is a lot less proven than that of batteries, which have been doing hundreds of thousands of cycles in hybrids for over a decade.